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Lodish H, Berk A, Zipursky SL, et al. Molecular Cell Biology. 4th edition. New York: W. H. Freeman; 2000.

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Molecular Cell Biology. 4th edition.

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Section 21.1Overview of Neuron Structure and Function

In this introductory section, we describe the structural features that are unique to neurons and the types of electric signals that they use to process and transmit information. We then introduce synapses, the specialized sites where neurons send and receive information from other cells, and some of the circuits that allow groups of neurons to coordinate complex processes. Each of these topics will be covered in more detail in later sections of the chapter.

Specialized Regions of Neurons Carry Out Different Functions

Although the morphology of various types of neurons differs in some respects, they all contain four distinct regions with differing functions: the cell body, the dendrites, the axon, and the axon terminals (Figure 21-1).

Figure 21-1. Structure of typical mammalian neurons.

Figure 21-1

Structure of typical mammalian neurons. Arrows indicate the direction of conduction of action potentials in axons (red). (a) Multipolar interneurons. Each has profusely branched dendrites, which receive signals at synapses with several hundred other neurons, (more...)

The cell body contains the nucleus and is the site of synthesis of virtually all neuronal proteins and membranes. Some proteins are synthesized in dendrites, but no proteins are made in axons and axon terminals, which do not contain ribosomes. Proteins and membranes that are required for renewal of the axon and nerve termini are synthesized in the cell body and assembled there into membranous vesicles or multiprotein particles. By a process called anterograde transport, these materials are transported along microtubules down the length of the axon to the terminals, where they are inserted into the plasma membrane or other organelles (Chapter 19). Axonal microtubules also are the tracks along which damaged membranes and organelles move up the axon toward the cell body; this process is called retrograde transport. Lysosomes, where such material is degraded, are found only in the cell body.

Almost every neuron has a single axon, whose diameter varies from a micrometer in certain nerves of the human brain to a millimeter in the giant fiber of the squid. Axons are specialized for the conduction of a particular type of electric impulse, called an action potential, outward, away from the cell body toward the axon terminus. An action potential is a series of sudden changes in the voltage, or equivalently the electric potential, across the plasma membrane (Figure 21-2a). When a neuron is in the resting (nonstimulated) state, the electric potential across the axonal membrane is approximately −60 mV (the inside negative relative to the outside); the magnitude of this resting potential is similar to that of the membrane potential in most non-neuronal cells (Chapter 15). At the peak of an action potential, the membrane potential can be as much as +50 mV (inside positive), a net change of ≈110 mV. This depolarization of the membrane is followed by a rapid repolarization, returning the membrane potential to the resting value. These characteristics distinguish an action potential from other types of changes in electric potential across the plasma membrane and allow an action potential to move along an axon without diminution.

Figure 21-2. (a) An action potential is a sudden, transient depolarization of the membrane followed by repolarization to the resting potential of about −60 mV.

Figure 21-2

(a) An action potential is a sudden, transient depolarization of the membrane followed by repolarization to the resting potential of about −60 mV. This recording of the axonal membrane potential in a presynaptic neuron shows that it is generating (more...)

Action potentials move rapidly, at speeds up to 100 meters per second. In humans, axons may be more than a meter long, yet it takes only a few milliseconds for an action potential to move along their length. An action potential originates at the axon hillock, the junction of the axon and cell body, and is actively conducted down the axon into the axon terminals, small branches of the axon that form the synapses, or connections, with other cells (Figure 21-2b). A single axon in the central nervous system can synapse with many neurons and induce responses in all of them simultaneously.

Most neurons have multiple dendrites, which extend out-ward from the cell body and are specialized to receive chemical signals from the axon termini of other neurons. Dendrites convert these signals into small electric impulses and transmit them inward, in the direction of the cell body. Neuronal cell bodies can also form synapses and thus receive signals (Figure 21-3). Particularly in the central nervous system, neurons have extremely long dendrites with complex branches. This allows them to form synapses with and receive signals from a large number of other neurons, perhaps up to a thousand. Electric disturbances generated in the dendrites or cell body spread to the axon hillock. If the electric disturbance there is great enough, an action potential will originate and will be actively conducted down the axon.

Figure 21-3. Typical interneurons from the hippocampal region of the brain makes about a thousand synapses.

Figure 21-3

Typical interneurons from the hippocampal region of the brain makes about a thousand synapses. The cells were stained with two fluorescent antibodies: one specific for the microtubule-associated protein MAP2 (green), which is found only in dendrites and (more...)

Synapses Are Specialized Sites Where Neurons Communicate with Other Cells

Synapses generally transmit signals in only one direction: an axon terminal from the presynaptic cell sends signals that are picked up by the postsynaptic cell (see Figure 21-2b). There are two general types of synapse: the relatively rare electric synapse, discussed later, and, the chemical synapse, illustrated in Figure 21-4. In this type of synapse, the axon terminal of the presynaptic cell contains vesicles filled with a particular neurotransmitter. The postsynaptic cell can be a dendrite or cell body of another neuron, a muscle or gland cell, or, rarely, even another axon. When an action potential in the presynaptic cell reaches an axon terminal, it induces a localized rise in the level of Ca2+ in the cytosol. This, in turn, causes some of the vesicles to fuse with the plasma membrane, releasing their contents into the synaptic cleft, the narrow space between the cells. The neurotransmitters diffuse across the synaptic cleft; it takes about 0.5 millisecond (ms) for them to bind to receptors on postsynaptic cells.

Figure 21-4. A chemical synapse.

Figure 21-4

A chemical synapse. (a) A narrow region —  the synaptic cleft — separates the plasma membranes of the presynaptic and postsynaptic cells. Transmission of electric impulses requires release of a neurotransmitter (more...)

Binding of the neurotransmitter triggers changes in the ion permeability of the postsynaptic plasma membrane, which, in turn, changes the membrane’s electric potential at this point. If the postsynaptic cell is a neuron, this electric disturbance may be sufficient to induce an action potential. If the postsynaptic cell is a muscle, the change in membrane potential following binding of the neurotransmitter may induce contraction; if a gland cell, the neurotransmitter may induce hormone secretion. In some cases, enzymes attached to the fibrous network connecting the cells destroy the neurotransmitter after it has functioned; in other cases, the signal is terminated when the neurotransmitter diffuses away or is transported back into the presynaptic cell.

The postsynaptic neuron at certain synapses also sends signals to the presynaptic one. Such retrograde signals can be gases, such as nitric oxide and carbon monoxide, or peptide hormones. This type of signaling, which modifies the ability of the presynaptic cell to signal the postsynaptic one, is thought to be important in many types of learning.

Neurons Are Organized into Circuits

In complex multicellular animals, such as insects and mammals, various types of neurons form signaling circuits. In the simple type of circuit called a reflex arc, interneurons connect multiple sensory and motor neurons, allowing one sensory neuron to affect multiple motor neurons and one motor neuron to be affected by multiple sensory neurons; in this way interneurons integrate and enhance reflexes. For example, the knee-jerk reflex in humans involves a complex reflex arc in which one muscle is stimulated to contract while another is inhibited from contracting (Figure 21-5). Such circuits allow an organism to respond to a sensory input by the coordinated action of sets of muscles that together achieve a single purpose. However, such simple nerve systems do not directly explain higher-order brain functions such as reasoning and computation.

Figure 21-5. The knee-jerk reflex arc in the human.

Figure 21-5

The knee-jerk reflex arc in the human. Positioning and movement of the knee joint are accomplished by two muscles that have opposite actions: Contraction of the quadriceps muscle straightens the leg, whereas contraction of the biceps muscle bends the (more...)

The sensory and motor neurons of circuits such as the knee-jerk reflex are contained within the peripheral nervous system (Figure 21-6). These circuits send information to and receive information from the central nervous system (CNS), which comprises the brain and spinal cord and is composed mainly of interneurons. Highly specialized sensory receptor cells, which respond to specific environmental stimuli, send their outputs either directly to the brain (e.g., taste and odorant receptors) or to peripheral sensory neurons (e.g., pain and stretching receptors). The peripheral nervous system contains two broad classes of motor neurons. The somatic motor neurons stimulate voluntary muscles, such as those in the arms, legs, and neck; the cell bodies of these neurons are located inside the central nervous system, in either the brain or the spinal cord. The autonomic motor neurons innervate glands, heart muscle, and smooth muscles not under conscious control, such as the muscles that surround the intestine and other organs of the gastrointestinal tract. The two classes of autonomic motor neurons, sympathetic and parasympathetic, generally have opposite effects: one class stimulates a muscle or gland, and the other inhibits it. Somatic sensory neurons, which convey information to the central nervous system, have their cell bodies clustered in ganglia, masses of nerve tissue that lie just outside the spinal cord. The cell bodies of the motor neurons of the autonomic nervous system also lie in ganglia. Each peripheral nerve is actually a bundle of axons; some are parts of motor neurons; others are parts of sensory neurons.

Figure 21-6. A highly schematic diagram of the vertebrate nervous system.

Figure 21-6

A highly schematic diagram of the vertebrate nervous system. The central nervous system (CNS) comprises the brain and spinal cord. It receives direct sensory input from the eyes, nose, tongue, and ears. The peripheral nervous system (PNS) comprises three (more...)

Having surveyed the general features of neuron structure, interactions, and simple circuits, let us turn to the mechanism by which a neuron generates and conducts electric impulses.

SUMMARY

  •  The cell body of a neuron contains the nucleus and lysosomes and is the site of synthesis and degradation of virtually all neuronal proteins and membranes.
  •  Axons are long processes specialized for the conduction of action potentials away from the neuronal cell body.
  •  Action potentials are sudden membrane depolarizations followed by a rapid repolarization. They originate at the axon hillock and move toward axon terminals, where the electric impulse is transmitted to other cells via an electric or chemical synapse (see Figure 21-2).
  •  Most neurons have multiple dendrites, which receive chemical signals from the axon termini of other neurons.
  •  When an action potential reaches a chemical synapse, a neurotransmitter is released into the synaptic cleft. Binding of the neurotransmitter to receptors on the postsynaptic cell changes the ion permeability and thus the electric potential of the postsynaptic plasma membrane (see Figure 21-4).
  •  Neurons are organized into circuits. In a reflex arc, such as the knee-jerk reflex, interneurons connect multiple sensory and motor neurons, allowing one sensory neuron to affect multiple motor neurons. One muscle can be stimulated to contract while another is inhibited from contracting (see Figure 21-5).
Image ch21f31

By agreement with the publisher, this book is accessible by the search feature, but cannot be browsed.

Copyright © 2000, W. H. Freeman and Company.
Bookshelf ID: NBK21535

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